Stevens Chap12 Cast

8/3/2019 Stevens Chap12 Cast

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CSCI 5273

Computer Networks

Stevens, Chapter 12 & 13Broadcast & Multicast

with IGMP

Dirk Grunwald

Assoc. ProfessorDept. of Computer Science

University of Colorado, Boulder


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Using Broadcast

u UDP broadcast involves sending to explicit broadcastaddresses

l Most POSIX implementations require you explic itly enablebroadcast

ret = setsockopt(sockfd, SOL_SOCKET,SO_BROADCAST, &on, sizeof(on));

u Only applicable to UDP!


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Broadcast Addresses

u Limited Net Broadcast - 255.255.255.255

l Never forwarded by a router!

u Net-directed Broadcast - e.g., 128.138.255.255l A router must forward a net-directed broadcast, but must have

an option to disable this.

u Subnet-directed Broadcast - e.g., 128.138.202.255


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Multicast

u Class D addresses are mult icast addresses

l 224.0.0.0 through 239.255.255.255

uA specific multicast address defines a network group

u Two special network groups

l

224.0.0.xxx is never routedl 224.0.0.1 - all hosts group

l 224.0.0.2 - all routers group


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Well-defined multicast groups

u ntp.mcat.net is 224.0.1.1

l Network time protocol


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////

// Set socket option to to joint// Set socket option to to joint mcastmcast

////

{{

struct ipstruct ip__mreq mreqmreq mreq;;

memcpymemcpy(&(&mreqmreq..imrimr__multiaddrmultiaddr,,

&from_&from_addraddr.sin_.sin_addraddr.s_.s_addraddr,,

sizeofsizeof((structstruct in_in_addraddr));));

mreqmreq..imrimr_interface.s__interface.s_addraddr == htonlhtonl(INADDR_ANY);(INADDR_ANY);

retret == setsockoptsetsockopt((sockfdsockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP,, IPPROTO_IP, IP_ADD_MEMBERSHIP,

&&mreqmreq,, sizeofsizeof((mreqmreq));));

check_and_exit(check_and_exit(retret, ", "setsockoptsetsockopt");");

}}

Joining a Multicast Group prior to receive

Desired multicast

group


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Using TTL to define multicast scope

u TTL field is used to limit propogation of mult icast packets

u In IPv4

l 0 - node local - doesnt leave machine

l 1 - link local - doesnt get routedl


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////

////

// Set socket option to to joint// Set socket option to to joint mcastmcast

////

{{

u_charu_char ttlttl = 16;= 16;

retret == setsockoptsetsockopt((sockfdsockfd, IPPROTO_IP, IP_MULTICAST_TTL,, IPPROTO_IP, IP_MULTICAST_TTL,

&&ttlttl,, sizeofsizeof((ttlttl));));

check_and_exit(check_and_exit(retret, ", "setsockoptsetsockopt");");

}}

Setting a TTL scope

Desired TTL

field


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Administrative Scoping

u 239.xxx.yyy.zzz is the administratively scope mult icast IPspace

l Addresses assigned locally to an organization, but not uniqueacross organizations

l Border routers must not forward

u link-local -- 224.0.0.0 to 224.0.0.255

u site-local -- 239.255.0.0 to 239.255.255.255

u organization-local - 239.192.0.0 to 239.195.255.255u global -- 224.0.1.0 to 238.255.255.255


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Converting Multicast to Ethernet

u Multicast addresses are targeted to a number of clients

l How does the ethernet card know which messages to receive?

u Could simple broadcast all packets

l Takes the same amount of network bandwidth as selectivemulticast, but..

l Disturbs all machines

u Can use ARP to advertise single MAC as resolving

multiple IP addressesl ..But multiple machines want to receive


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u Ethernet cards can usually receive on mult iple MACaddresses

u Multicast router enters a virtual MAC address, clients

receive on that virtual MAC

1110 0000

00101 11100000 00000000 0001

Mapping a Multicast Address toEthernet Address


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Digression - Virtual IP Addresses

u alias alias_address[/bitmask]

l Establishes an additional network address for this interface.

l Eample:ifconfig eth0 alias 128.138.241.79/26

u The following aliaslist command adds network addresses40 through 50, inclusive, to subnets

18.240.32, 18.240.64, and 18.240.96

ifconfig aliaslist 18.240.32,64,96.40-50

u Doesnt require multiple MAC addresses, but oftenimplemented using them.


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IGMP - Internet Group ManagementProtocol

u Part of IP layer

u Lets hosts & routers know who belongs to what groups

IP Header

(20 bytes)

IGMP

Message (8 bytes)


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IGMP Message Format

Version(1) Type(1-2) MBZ 16-bit checksum

32-bit group address (Class D IP address)

u Typel 1 is a query sent by multicast router

l 2 is a reponse sent by a host

u Group address is a class D IP address

l On query, its zero

l On response, its the group address being reported


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IGMP Host Reports

Host Router

u Host sends a report when it joins a group

u Doesnt report when it leaves the group, but doesnt respond tonext query

IGMP report, TTL =1IGMP group addr = group address

dest IP addr = group address

src IP addr = hosts IP address


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IGMP Router Query

Host Router

u Router sends query at regular intervals to see if anyone sti llbelongs to anygroups. Queries sent out each interface.

u Host responds by sending one responsefor each group to which it belongs IGMP query, TTL =1

IGMP group addr = 0.0.0.0

dest IP addr = 224.0.0.1

src IP addr = routers IP address


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Sample Query on Windows Bootup

05:52:32.517937 arp who-has 192.168.1.6 tell

192.168.1.6

05:52:32.518010 linux > 192.168.1.6: icmp: echo

request

05:52:33.378928 192.168.1.6 > ALL-ROUTERS.MCAST.NET:

icmp: router solicitation

05:52:37.511385 arp who-has 192.168.1.6 tell linux

05:52:37.511664 arp reply 192.168.1.6 is-at

0:a0:cc:3b:95:4b

05:52:38.453193 192.168.1.6 > ALL-ROUTERS.MCAST.NET:

icmp: router solicitation05:52:43.477432 192.168.1.6 > ALL-ROUTERS.MCAST.NET:

icmp: router solicitation


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Multicast Routes


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Sender Sends DatagramWith Specified TTL

Pruned

because no

one is

listening


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Receiver Starts Joins group


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Routers Form Destination Tree


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Non-participants prune themselves

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Stevens Chap12 Cast